Fatigue Life Analysis of Dented Pipes Subjected to Internal Pressure

Abstract

Steel pipelines that transport pressurized fluids can contain a dent defect which is a permanent deformation of the outer wall due to an impact with a foreign body. This defect induces a high local stress concentration. This paper presents an optimized numerical model based on finite elements to assess the stress concentration factor around constrained and unconstrained dents in an API X52 steel pipe subjected to internal pressure. The simulation of the finite element model is conducted in two phases; the first phase concerns the realization of the dent, while the second one takes as initial geometry the dented pipe and subjects it to an internal pressure. The validation results of this finite element model show that it can precisely provide Von Mises stress around the defect needed to calculate the stress concentration factor. The fatigue life is then estimated based on S-N curves with the Gerber criterion to account for the mean stress effect for various pipe geometries, dent depths and dent types. These dents are generated using spherical and rectangular indenters in longitudinal and transverse orientations under constrained and unconstrained configurations. A parametric study conducted on 150 cases of dented pipes subjected to internal pressure between 0% and 50% MOP (maximum operating pressure) enabled us to analyze the effect of the various types of dents on the integrity of the structure.